Mineral processing plants often utilise a flotation process to separate the finely-ground particulates of a required mineral from waste rock. This is achieved in a flotation tank or cell in which the slurry is placed and to which fine air bubbles and reagents are added. The tank is then agitated and the resulting froth which rises to the top of the flotation cell has the fine particles of the required mineral adhering to the air bubbles. Collection of the froth then provides a means of collecting the required concentrated mineral which has been extracted by the process.
The froth from the flotation process contains the required mineral and normally must be pumped to the next processing stage. The different types of froth produced depend a lot on the particles sizes being floated, the type and quantity of reagents and the quantity and size of the air bubbles. The froth process is continuous but there is currently no commercially-available equipment which can reduce the air content of the froth, and it is not practical to leave the froth standing until the air separates by itself before pumping the remanent particles and liquid which formed the froth.
To achieve good recovery results from the flotation process requires that the mineral ore be ground to very fine particle sizes (in some cases less than 10 micrometres). Also to achieve good mineral recovery the reagents used in the process need to be controlled, but quite often this, combined with the amount of bubbles necessary to make the process efficient, can result in a very stable and tenacious froth. These tenacious froths when left in a vessel would typically take 12 to 24 hours to reduce to the water and solid state only, that is, the bubbles would be extremely slow to disperse.
Pumps for use for pumping froth currently are in the form of vertical and/or horizontally disposed pumps. Vertical pumps are arranged so that the pump inlet is disposed generally vertically, and horizontal pumps are arranged with the pump inlet disposed generally horizontally. Vertical froth pumps have been demonstrated to be able to pump very tenacious froths, but are often physically quite large and therefore must be considered in the initial design of a mineral processing plant.
Horizontal pumps, on the other hand, have also been used for froth pumping applications, but these are not always successful with tenacious froths. Horizontal pumps have traditionally been deliberately oversized in froth-handling applications. A larger size pump means that they can be operated inefficiently with a resultant low flow and a high air entrainment due to the froth. Mechanical failures can become a problem with such unsteady pumping. Froth is full of air, but being present as very small bubble sizes has less effect than the same quantity of air in the form of large bubbles. However, there is a point at which the ability of a pump to tolerate froth will drop due to the effect of the air. The air tolerance of a pump is also related to the net positive suction head (NPSH) characteristic; that is, the lower the net pressure available at the intake to the pump, the more likely it is that the performance will be affected.
Pumps have been developed to specifically handle frothy fluids of this type. During the pumping operation, the heavier fraction of the fluid migrates to an outer region of the pump and a lighter fraction tends to migrate towards an inner region. There is a need to be able to efficiently remove the lighter fraction.